We study turbulent channel flow of a binary mixture of finite-sized neutrally buoyant rigid particles by means of interface-resolved direct numerical simulations. We fix the bulk Reynolds number and total solid volume fraction, Reb=5600 and Φ=20% , and vary the relative fraction of small and large particles. The binary mixture consists of particles of two different sizes, 2h/dl=20 and 2h/ds=30 where h is the half-channel height and dl and ds the diameters of the large and small particles. While the particulate flow statistics exhibit a significant alteration of the mean velocity profile and turbulent fluctuations with respect to the unladen flow, the differences between the mono-disperse and bi-disperse cases are small. However, we observe a clear segregation of small particles at the wall in binary mixtures, which affects the dynamics of the near-wall region and thus the overall drag. This results in a higher drag in suspensions with a larger number of large particles. As regards bi-disperse effects on the particle dynamics, a non-monotonic variation of the particle dispersion in the spanwise (homogeneous) direction is observed when increasing the percentage of small/large particles. Finally, we note that particles of the same size tend to cluster more at contact whereas the dynamics of the large particles gives the highest collision kernels due to a higher approaching speed.

Inertial regimes in a channel flow of suspension of finite-size neutrally buoyant particles are studied for a wide range of Reynolds numbers: 500 Re 5000, and particle volume fractions: 0 0:3. The flow is classified in three different regimes according to the phase-averaged stress budget across the channel [2]. The laminar viscous regime at low Re and - where the viscous stress is the dominating term in the budget, the turbulent regime at high Re and relatively low where the momentum is mainly transferred by the action of the Reynolds stress and the inertial shear-thickening regime where the particle stress contributes the most to the significant enhancement of the wall shear stress. Particle distribution and dispersion properties provide additional evidence for the existence of the three different regimes.